1. Field of Invention
The present invention relates to methods that have been developed for producing high-performance silicon carbide (SiC) fibers, SiC multi-fiber tows, SiC fiber architectural preforms, and high-temperature SiC fiber-reinforced composite structures with state-of-the-art thermostructural and environmental performance. In particular, the present invention describes thermal-chemical treatment processes for fiber architectural preforms formed from high-strength SYLRAMIC SiC fibers that were commercially produced at high temperature (>1600° C.) using boron-containing sintering aids. The treated SiC fibers have been shown to display state-of-the-art properties in terms of tensile strength, creep resistance, and rupture resistance. The treated preforms have been shown to display enhanced shape capability and to yield SiC fiber-reinforced ceramic matrix composite structures with state-of-the-art properties in terms of intrinsic temperature capability, ultimate tensile strength, creep strength, rupture strength, and strength retention after exposure to elevated and high-temperature oxidizing conditions.
2. Description of Related Art
The realization of improved gas turbine engines for aero and space propulsion as well as for land-based power generation will depend strongly on advancements made in the upper use temperature and life capability of the structural materials used for such engine hot-section components as combustor liners, inlet turbine vanes, and turbine blades. Components with improved thermal capability and longer life between maintenance cycles will allow improved engine performance by reducing cooling requirements and life-cycle costs. This in turn is expected to reduce fuel consumption, NOx and CO2 emissions, ticket cost, and flight times for commercial aircraft; to allow improved thrust-to-weight and performance for space and military engines; and to reduce emissions and power costs for the electrical power industry.
Today the major thrust in the United States, Japan, and Europe for achieving these benefits is by the development of fiber-reinforced ceramic matrix composites (CMC) in general and of silicon-carbide fiber-reinforced silicon-carbide matrix (SiC/SiC) composites in particular. These materials are not only lighter in weight and capable of higher use temperatures than state-of-the-art metallic alloys, but also capable of providing significantly better damage tolerance than un-reinforced monolithic ceramics. However, for successful application in advanced gas turbine engines, the ceramic composites should be producible in multiple shapes and display and maintain the highest tensile strength possible at the service temperatures, environments, and lifetimes required for the hot-section components. Also, because of possibility of large thermal gradients in these components, the composites should provide the uppermost in thermal conductivity and creep resistance in order to minimize the development of thermal stresses and creep-induced dimensional changes within the materials during their service life.
Material science theory and ceramic composite experience have shown that chemical and physical conditions within the bulk and on the surface of the reinforcing ceramic fiber are the primary factors controlling such key CMC property requirements as high shape-ability, high as-produced tensile strength, and high strength retention during composite service under aggressive environments at high temperatures. These fiber-related factors have also been demonstrated to play an important role in other key CMC property requirements such as high thermal conductivity and high creep resistance. Thus the major technical challenge for implementation of ceramic composites in engine hot-section components is to develop ceramic fibers that can provide the uppermost in these properties, not only after fiber production, but also after CMC component fabrication and during engine service.
Prior art fiber materials for high-performance SiC/SiC composites include various commercially available polycrystalline SiC-based fiber types produced in continuous length by polymer spinning, curing, pyrolysis, and sintering. For reinforcement of SiC/SiC components, typical property requirements for the as-produced individual SiC fibers are high as-produced tensile strength (>2.5 GPa) and small diameter (<15 μm). The small diameter is required so that conventional textile forming processes could be used to produce net-shape fiber architectural preforms needed for CMC component shape and structural requirements. First generation SiC-based fiber types that have met these requirements include the non-stoichiometric (C/Si>1) NICALON fiber from Nippon Carbon and the TYRANNO Lox M fiber from UBE Industries. Besides being carbon rich, these fibers contain small (<5 nm) grains and high oxygen content which contribute to grain growth, excessive grain-boundary sliding, and chemical decomposition, thereby limiting fiber thermal conductivity, creep-rupture resistance, and capability for strength retention to composite fabrication temperatures less than 1300° C. and to composite long-term service temperatures less than 1200° C.
Production methods for second generation SiC fiber types, such as the HI-NICALON fiber from Nippon Carbon, have focused primarily on reducing oxygen content, but the remaining small grains and large carbon content still limit composite long-term service temperatures to less than 1300° C., as well as giving rise to non-optimized fiber thermal conductivity. Production methods for the more recent SiC fiber types have added high-temperature sintering processes that yield larger grains and purer, more stoichiometric (C/Si ˜1) compositions. These include the SYLRAMIC fiber from Dow Corning, the HI-NICALON Type S fiber from Nippon Carbon, and the TYRANNO SA fiber from UBE Industries. The reduced oxygen and carbon content allow these near-stoichiometric fiber types to maintain tensile strength at fiber production temperatures above 1600° C., which are much higher in comparison to those used for the earlier generation types. The higher production temperatures in turn allow the grains to grow and provide higher fiber creep resistance and thermal conductivity, provided grain boundaries with high purity can be achieved.
Some important microstructural and physical properties of the most thermally capable commercial SiC fiber types in their as-produced condition are listed in the table in FIG. 1. The fibers are generally made available in two architectural forms: (1) one-dimensional continuous lengths of multi-fibers or “tows’ that typically contain 500 to 800 fibers and can be easily handled and formed into component-specific three-dimensional architectural preforms by end-users, and (2) two-dimensional planar cloth or fabric in which the tows are typically woven in two orthogonal directions (0/90) for laminate construction of simple-shaped components. For comparison purposes, FIG. 1 also includes the developmental SYLRAMIC-iBN and SYLRAMIC-iC fibers, which are examples of high-performance SiC fiber types that have been produced from the SYLRAMIC fiber using the methods of this invention.
For the purpose of achieving high performance high-temperature SiC/SiC components, ceramic composite experience has also shown that a variety of issues exist which relate to retaining the as-produced properties of the reinforcing SiC fibers during component fabrication and service. Many of these issues arise in the fabrication stage during the various steps of (1) shaping the continuous length fibers into architectural arrays or preforms that yield near net-shape component structures, (2) coating the fibers within the architectural preforms with thin fiber coatings or interphase materials that are required for matrix crack deflection, and (3) infiltrating the coated-fiber architectural preforms with SiC-based matrix material, which is often performed at temperatures of 1400° C. and above.
For example, during the architecture formation or “preforming” step, potential fiber strength degrading mechanisms include fiber bending, which can introduce detrimental residual stresses in the fibers, and fiber-fiber abrasion which may weaken the fibers by providing new surface flaws. In combination, these mechanisms could cause premature fiber fracture during the preforming step or eventually during component structural service.
Also, during the fiber coating or interphase formation step, which is typically performed by the chemical vapor infiltration (CVI) of boron nitride (BN) or carbon (C) yielding precursor gases, potential fiber strength degrading mechanisms include the risk that chemically aggressive gases such as halogens, hydrogen, and oxygen may reach the SiC fiber surface before the protective and non-reactive BN and C interphase materials are formed. The halogens and hydrogen have been demonstrated to cause fiber weakening by surface flaw etching; whereas oxygen allows the growth of silica on the fiber surfaces, which in turn causes strong mechanical bonds to be formed between contacting fibers in the fiber architectures. The detrimental consequence of fiber-fiber bonding is that if one fiber should fracture prematurely, all others to which it is bonded will prematurely fracture, causing composite fracture or rupture at stresses much lower than those that would be needed if the fibers were able to act independently. This oxidation issue is also serious during SiC/SiC service where the possibility exists that cracks may form in the SiC matrix, thereby allowing oxygen from the service environment to reach the reinforcing fibers. Because of the high reactivity of carbon with oxygen above ˜500° C. and subsequent volatility of the bi-products, cracking of the matrix can be especially serious for those SiC fiber types with carbon-rich surfaces or for fibers and interphase materials based on carbon.
Finally, during the matrix formation step, current SiC/SiC fabrication trends are progressing toward SiC-based matrices that are processed at 1400° C. and above in order to improve matrix and composite creep-rupture resistance and thermal conductivity. In these cases, the matrix formation times and temperatures are high enough to cause strength degradation in the non-stoichiometric SiC fibers that are produced at temperatures below 1400° C. Strength degradation can also occur in a near-stoichiometric type if its maximum production temperature (see FIG. 1) is below that for matrix processing.
Based on achieving SiC/SiC components that display the highest temperature capability and highest thermostructural properties after fabrication and during service, current state-of-the-art SiC/SiC fabrication routes are employing the following constituent materials: (1) commercial SiC fiber types with high as-produced tensile strength, carbon-free surfaces, and production temperatures above 1600° C.; (2) BN-based interphases, which are significantly more oxidation resistant than carbon-based materials; and (3) SiC-based matrices with high creep-rupture resistance, high thermal conductivity, and formation temperatures above 1400° C. As indicated in FIG. 1, the commercial SiC fiber type that meets most of these fiber requirements in its as-produced condition is the SYLRAMIC fiber that was originally produced by Dow Corning and is currently being produced by COI Ceramics. This fiber type is fabricated by the polymer route, in which precursor fibers based on polycarbosilane are spun into multi-fiber tows and then cured, pyrolyzed, and sintered at high temperature (>1700° C.) using boron-containing sintering aids (U.S. Pat. Nos. 5,071,600, 5,162,269, 5,268,336, 5,279,780, and Ceram. Eng. Sci. Proc., Vol. 18[3], 1997, pp. 147-157). The sintering process results in very strong fibers (>3 GPa) that are dense, oxygen-free, near stoichiometric, and contain ˜1 and ˜3 weight % of boron and TiB2, respectively.
Despite displaying enhanced properties, performance issues related to certain factors existing in the as-produced bulk and on the fiber surface have been found to limit the thermostructural performance of the SYLRAMIC fiber, both as individual fibers and as textile-formed architectural preforms for SiC/SiC composites. For example, excess boron in the bulk is typically located on the fiber grain boundaries, thereby inhibiting the fiber from displaying the optimum in creep resistance, rupture resistance, and thermal conductivity associated with its grain size. Also in the presence of oxygen-containing environments during composite fabrication or service, boron on the fiber surface can promote detrimental silica (SiO2) growth that bonds neighboring fibers together and degrades composite strength.
In addition, like all near-stoichiometric SiC fibers, the high elastic modulus of the SYLRAMIC fiber (˜400 GPa), gives rise to elastic tensile stresses on the fiber surfaces when the fibers are bent and shaped to form simple 2D fabric or more complex 3D component architectural preforms. These stresses limit fiber formability and add to any tensile stresses that are applied to the final composite component, thereby limiting component capability for resisting external stresses during service. Finally surface roughness exists on all SiC fibers, which can be correlated in magnitude to the fiber average grain size (see FIG. 1). Thus the near-stoichiometric SiC fibers, like the SYLRAMIC fiber, display the greatest surface roughness, which not only can cause adverse fiber-fiber abrasion during tow handling and shaping into complex architectures, but also can lead to adverse mechanical interlocking between contacting fibers within tows in the final composite microstructure. This interlocking effect is similar to the oxide-bonding effect where the failure of one weak fiber can cause the premature failure of its strong neighboring fibers, resulting in poor composite ultimate strength and toughness.